Abstract: Objective 430 ferritic stainless steel heat exchangers were brazed in continuous mesh belt brazing furnace with ammonia decomposition shielding gas. The influential regulation of residual ammonia in ammonia decomposition shielding gas on the microstructure and hardness of stainless steel was studied. Methods The microstructure evolution, element diffusion and precipitated phase formation during the brazing process of 430 stainless steel were investigated by optical microscopy, electron microscopy, energy dispersive spectrometer and hardness tester, as well as their influence on hardness of stainless steel. Results The results show that microstructure of brazed stainless steel in non-residual ammonia shielding gas was composed of fine ferrite grains outside and coarse ferrite grains inside. N₂ in shielding gas dissociated and diffused to the surface grain boundaries to form nitrogen-containing compounds that hindered grain growth. The hardness after brazing increased from 140 HV of unbrazed to 180 HV. Brazing stainless steel in shielding gas with residual ammonia of 0.59 mg/m³, thermal decomposition of residual ammonia caused nitriding of stainless steel, resulting in the formation of a martensitic hardening layer on the surface and a mass Cr₂N phases within the coarse ferrite grains, and hardness of the brazed stainless steel increased to 535 HV. Plastic processing increased nitriding amount of stainless steel during brazing, resulting in the formation of more martensite and larger intragranular precipitated phases in the brazed stainless steel. Conclusion The nitrogen atoms generated by thermal decomposition of residual ammonia in shielding gas caused nitriding of stainless steel, resulting in an abnormal increase in hardness of 430 stainless steel heat exchanger flange. Reducing the residual ammonia content in shielding gas was an effective measure to suppress abnormal increase of hardness of heat exchanger flange.